TiO2 composite membranes

TiO2 composite membranes

Applied Surface Science 254 (2008) 7080–7086 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/lo...

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Applied Surface Science 254 (2008) 7080–7086

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Preparation and characterization of PES/TiO2 composite membranes Guiping Wu a,*, Shuying Gan b, Longzhe Cui a, Youyi Xu c a Key Laboratory for Catalysis and Material Science of Hubei Province, College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan 430074, PR China b Shanghai Academy of Environmental Sciences, Shanghai 20023, PR China c Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 21 August 2007 Received in revised form 27 March 2008 Accepted 12 May 2008 Available online 16 May 2008

Polyethersulfone (PES)/TiO2 composite membranes were prepared by phase inversion method with nano-TiO2 as additive. The influence of TiO2 on the morphologies and the performances of PES/TiO2 membranes were investigated through the methods of SEM, XRD, TGA, contact angle goniometer, mechanical strength tests and filtration experiments. The results showed that the structure of membrane was not obviously affected by addition of TiO2, and the performances such as hydrophilicity, thermal stability, mechanical strength and anti-fouling ability of membrane were enhanced through adding TiO2 nanoparticles. At 0.5 wt.% TiO2 content, the composite membrane has an excellent performance, however higher TiO2 content (than 0.5 wt.%) resulted in defective pore structure of the membranes and decline of the performances, such as permeability and mechanical strength. TGA and mechanical strength analyses indicated good compatibility between polymers and TiO2 nanoparticles. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Polyethersulfone TiO2 Anti-fouling Composite membrane

1. Introduction Polyethersulfone (PES) is a kind of special engineering plastic. It possesses many good performances such as high mechanical property and heat distortion temperature, good heat-aging resistance and environmental endurance as well as easy processing. It has become an important separation membrane material, but its hydrophobicity controlled by PES structure leads to a low membrane flux and poor anti-fouling property, which has great effect on its application and usage life [1]. It is necessary to modify the PES membrane surface by physical or chemical methods in order to improve its hydrophilicity. Recently, modification methods of membrane involve ultraviolet irradiation [2], blending with hydrophilic materials [3], graft polymerization [4], plasma graft [5], and so on. Among these methods, blending with inorganic materials, especially nanoparticles, has attracted much interest due to their convenient operation and mild conditions [6]. Moreover, by the way of blending, the modified membrane can combine basic properties of organic and inorganic materials and offer specific advantages for the preparation of artificial membranes with excellent separation performances, good thermal and

* Corresponding author. Tel: +86 13971206312; fax: +86 27 6784 752. E-mail address: [email protected] (G. Wu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.221

chemical resistance and adaptability to the harsh environments, as well as membrane forming ability [7–10]. Nanoparticles that have been used to modify organic membrane include SiO2, TiO2, Al2O3, and so on [11]. Among different nanoparticles, TiO2 had received most attention because of its stability, availability, and promise for applications such as painting, catalysis and photocatalysis, battery, cosmetic, etc. [12]. In this paper, pure PES membrane and PES/TiO2 composite membrane were prepared by phase inversion methods. The morphology and hydrophilicity of membrane were characterized by scanning electron microscopy (SEM) and contact angle goniometer, respectively. X-ray diffraction and thermogravimetric analysis (TGA) were employed to analyze properties of membrane. To examine the anti-fouling and fouling mitigation abilities of membranes, a filtration experiment was also carried out. 2. Experimental 2.1. Materials Polyethersulfone was purchased from Jilin University (Changchun, China). Bovine serum albumin (BSA) was from Shanghai Bio Life Sci. & Technol. Co. Ltd. Rutile TiO2 nanoparticle (30 nm) was provided by Hangzhou Dayang Chemical Co. Ltd (Hangzhou, China). Other reagents were all AR grade.

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2.2. Membrane preparation At first, surface modification of TiO2 nanoparticles was performed to overcome their aggregation and to increase their dispersibility in the casting solution. The 5.0 g TiO2 nanoparticles and 0.1 g g-aminopropyl triethoxysilane were added to 100 mL ethanol. After stirring for 1 h under 70 8C, the solution was centrifuged. The modified TiO2 powders were washed with distilled water 3 times and dried below 80 8C. Pure PES membrane and PES/TiO2 composite flat membrane were prepared by phase inversion methods. Polyethersulfone (15 wt.%,), Polyvinylpyrrolidone (PVP, 5%), H2O (5 wt.%) and modified TiO2 (0, 0.3, 0.5, and 0.7 wt.%, respectively) were dissolved into N,N-dimethylacetamide (DMAc) solution and was stirred at 45 8C for 48 h to obtain a uniform and homogeneous casting suspension. The casting solution was cast with 100 mm casting knife onto a glass plate at room temperature. The nascent membrane was evaporated at 25 8C for 30 s, and then immersed in DMAc/water (20/80 in volume) coagulation bath. After complete coagulation, the membrane was transferred to a water bath for 1– 2 d at room temperature to remove the remaining solvent from the membrane structure. Then the membrane was dipped in ethanol for 24 h and in hexane for 48 h, and then dried in air.

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measured after the flux reached steady state and calculated with the following equation: V F¼ At where F is the pure water flux (L/m2 h), V, the permeate volume (l), A, the membrane area (m2) and t is the time (h). For each membrane, 5 samples were tested and the water flux was the average value of 5 times tests. The static contact angle between water and the membrane surface was measured to evaluate the membrane hydrophilicity, using a contact angle goniometer (Dataphysics, OCA20, Germany). To minimize experimental error, all samples were washed with distilled water to eliminate loosely adsorbed materials and then dried in a vacuum oven at 80 8C for 1 h. The reported contact angle was determined from the average value of 8 measurements. 2.3.5. Mechanical test The breaking strain and breaking strength of the membranes were examined to investigate the mechanical stability using AG-1 material test-machine (Shimadzu, Japan) at a loading velocity of 50 mm/min. The samples were shaped into 10 mm  100 mm and tested 10 times for each membrane.

2.3. Membrane characterization 2.3.1. Morphology observation The morphologies of the surfaces and the cross-sections of the membranes were examined by a scanning electron microscope (FEI, SIRion200, Netherlands). Image magnifications were 2000 and 5000 for cross-sectional and surface views, respectively. Cross-sections were prepared by fracturing the membranes in liquid nitrogen. All specimens were freeze-dried and coated with a thin layer of gold before observation. 2.3.2. Porosity calculation Porosity (Pr, %) was calculated as a function of the membrane weight.   m Pr ¼ 1   100% sdr where m is the weight, s the area, d the average thickness and r the density of the membrane. The porosity data was the average value for 5 times measurements for each membrane. 2.3.3. Membrane mean pore diameter determination The membrane mean pore diameters were measured by the water flow rate method. The mean pore diameter of the total membrane was calculated by the following formula. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2:9  1:75eÞ  ð8hlJÞ r¼ e  A  DP where e is the membrane porosity (%), l, the membrane thickness (m), h, water viscosity (Pa s), J, the water flux (m3/s), A, the filtration area (m2) and DP the trans-membrane pressure (Pa). The mean pore diameter was calculated for 5 times according to the obtained porosity and the water flux. 2.3.4. Membrane performances The water fluxes of the membranes were tested with a SCM-300 ultrafiltration cup (Ecologic Center, Shanghai) having 12.56 cm2 of membrane surface area. The membrane was first compacted for 30 min at 150 kPa to minimize compaction effects, and then the pressure was lowered to 100 kPa and the pure water flux was

2.3.6. XRD analysis Crystalline properties of the membranes were investigated by an X-ray diffractometer (Bruker D8, Germany) equipped with monochromatic Cu Ka radiation (l = 0.154 nm) operated at 40 mA and 40 kV from 58 to 808. 2.3.7. Thermal analysis The thermal stability of membranes was evaluated by thermogravimetric analysis (DSCQ100, TA Company, USA). The TGA measurements were carried out under nitrogen atmosphere at a heating rate of 10 8C/min from 20 to 800 8C. 2.4. Ultrafiltration experiments 2.4.1. Bovine serum albumin (BSA) solutions Bovine serum albumin solution (2 g/L) was prepared using 10 mM phosphate buffered saline (PBS) at pH 7.4 as solvent. Protein concentration was determined spectroscopically at 280 nm using a UV–vis spectrophotometer (Shimadzu UV–vis 2450, Japan). 2.4.2. Filtration system and protocol A dead-end filtration system (Fig. 1) was designed to characterize the filtration performance of PES/TiO2 composite membranes. The system consisted of a 300 mL ultrafiltration cup (SCM-300, Ecologic Center, Shanghai) connected to two pressure tanks filled with deionized water and BSA solution, respectively. The membrane area was 12.56 cm2. The system was pressured by high pressure diaphragmatic pump. All filtration experiments were conducted at a constant trans-membrane pressure of 100 kPa and a system temperature of 25  2 8C. A schematic representation of the filtration protocol is shown in Fig. 2. Each membrane was first compacted for 30 min at 150 kPa. The pressure was lowered to 100 kPa and the deionized water flux was measured (J0). The BSA solution was added to the pressure tank and the BSA solution flux was measured after a total of 40 mL of permeate were collected (Jp). Then the water flux was measured again (Jw). The membrane was cleaned with deionized water by filling and shaking the cell for 1 min thrice, and then the deionized water flux was measured (Jw1). The membrane was flipped over

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Fig. 1. Flow sheet for the dead-ended filtration system.

and deionized water filtered at 100 kPa for 30 s. The cell was then filled with deionized water and the pure water flux was measured (Jw2). 3. Results and discussion

during the membrane drying process when the TiO2 content was higher. During the drying, the shrinkage rates of the membrane and TiO2 were different, so the pores morphologies of the surface and the membrane interior were destroyed, resulting in the broken and collapsed pores.

3.1. Membrane morphology

3.2. Membrane properties

Surface and cross-sectional images of membranes prepared from PES/PVP/H2O/TiO2/DMAc systems with different contents of nano-TiO2 additive are shown in Fig. 3. The SEM photographs show that all prepared membranes were typical MF membranes, and highly porous surface and sponge-like cross-section could be obviously observed. There were an amount of TiO2 nanoparticles aggregates adsorbing or embedding uniformly on the outer surface and pore-surface of PES/TiO2 composite membrane (Fig. 3(b–d)) compared with pure PES membrane (Fig. 3(a)). This phenomenon indicates that the TiO2 nanoparticles dispersed uniformly into the membrane. Though the structure of the membranes were not obviously different with the increase of TiO2 content, the higher TiO2 content (0.7 wt.%) induced a slight aggregation phenomenon, and produced a considerable number of broken and collapsed pores mostly formed in the vicinity of TiO2 aggregates in membrane crosssection and on membrane surface. The defective pore structure originated from the interfacial stresses between polymer and TiO2

3.2.1. Hydrophilicity and permeability The hydrophilicity can be characterized with contact angle. The contact angle data of the membranes with different TiO2 content are listed in Table 1 which shows that the membrane hydrophilicity was improved by the addition of TiO2. This is due to the presence of TiO2 nanoparticles which contain a great deal of hydroxyl groups and amino groups, responsible for the hydrophilicity increase. The influence of TiO2 content on permeability was investigated through pure water flux experiments. The results show that the permeability of the membranes was enhanced with TiO2 increasing concentration, with a peak value at 0.5 wt.%. However, higher TiO2 concentration led to a decrease in permeability because of pore blockage caused by excessive TiO2 and pore collapse in the membrane cross-section. However, the difference in the membrane porosities with varying TiO2 content was inconspicuous.

Fig. 2. Filtration protocol: (a) to minimize compaction effects, deionized water was passed through the membrane for 30 min at a trans-membrane pressure (TMP) of 150 kPa, (b) TMP is lowered to 100 kPa and water flux (J0) is noted, (c) 2 g/L BSA solution is filtered at a TMP of 100 kPa until 40 mL of permeate is collected (Jp), (d) water flux is measured again (Jw), (e) the cell is rinsed with deionized water three times for 1 min each and the water flux measured (Jw1), and (f) the membrane is flipped over and deionized water filtered at 100 kPa for 30 s. The membrane is turned over to its original orientation cell and the water flux was measured (Jw2).

3.2.2. Thermal property The thermal analysis results of pure PES membrane and PES/ TiO2 composite membranes are illustrated in Fig. 4. It was found that the decomposition temperature (Td, defined as the temperature at 3% weight loss) increased with increasing amount of TiO2. This result further shows that the TiO2 nanoparticles were dispersed uniformly in the membrane and shows the good compatibility between TiO2 nanoparticles and PES. With increasing TiO2 content, more heat was absorbed by the TiO2 in the membranes during heating-up, the decomposition of PES was thus delayed, and the decomposition temperature of PES/TiO2 membrane was enhanced. 3.2.3. Mechanical strength The mechanical strength including breaking strain and breaking strength test results are listed in Table 2. It is clear that the mechanical strength of the membrane increased with the increase of the TiO2 concentration, especially, at 0.5 wt.% TiO2 concentration, the breaking strain and breaking strength reached a peak, respectively, and then declined with further increase of TiO2 concentration. These findings can be interpreted by the uniform

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Fig. 3. SEM photographs of PES/TiO2 composite membranes with different TiO2 contents (a) 0 wt.%, (b) 0.3 wt.%, (c) 0.5 wt.%, and (d) 0.7 wt.%; 1: cross-section, 2: top surface, and 3: bottom surface.

dispersion of the TiO2 nanoparticles in the membranes, which served as the physical cross-linkage to bear the stress of the load thus improving the membrane mechanical strength. However, an excessive TiO2 concentration may cause the nanoparticles

aggregation and decrease their dispersion in the polymeric membrane, leading to formation of defects and stress convergence points in the membrane under the loading force, thus weakened its mechanical stability. The maximum strain and breaking strength

Table 1 Performances of PES/TiO2 composite membranes with different TiO2 contents TiO2 content (wt.%)

Porosity (%)

Mean pore diameter (nm)

Water flux (L/m2 h)

Contact angle (8)

0 0.3 0.5 0.7

73.4 72.6 73.6 73.6

12.1 13.0 15.4 12.6

340 411 596 365

76.5 72.0 70.6 66.2

(1.5) (1.7) (1.4) (1.5)

Standard deviations were given in parentheses.

(0.2) (0.1) (0.1) (0.1)

(13) (12) (13) (13)

(1.4) (1.7) (1.2) (1.4)

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Fig. 4. TG curves of PES/TiO2 composite membranes with different TiO2 content: (a) 0 wt.%, Td 475.77 8C; (b) 0.3 wt.%, Td 479.32 8C; (c) 0.5 wt.%, Td 491.00 8C; (d) 0.7 wt.%, Td 500.20 8C. Table 2 Mechanical properties of PES/TiO2 composite membranes with different TiO2 contents TiO2 content (wt.%)

Breaking strain (%)

Breaking strength (MPa)

0 0.3 0.5 0.7

22.3 24.9 24.9 23.5

0.60 0.61 0.63 0.60

(0.5) (0.5) (0.6) (0.7)

(0.04) (0.06) (0.06) (0.05)

Standard deviations were given in parentheses.

increase 11.6% and 5.9%, respectively, at an appropriate amount of TiO2 nanoparticles. 3.2.4. XRD analyses The XRD patterns for TiO2 nanoparticles, PES membrane and PES/TiO2 composite membrane with 0.5 wt.% TiO2 content are shown in Fig. 5. The TiO2 nanoparticles had dominant peaks at 2u of 27.278, 36.068, 41.168 and 54.268. The PES/TiO2 hybrid membrane also had three crystalline characteristic peaks at 2u of 27.548, 37.168, 43.348 and 54.38, which was analogous with the characteristic peaks of TiO2 crystal powders in addition to the dispersion peak of amorphous PES, nevertheless their locations were right shifted slightly compared with that of TiO2 nanoparticles. The XRD results showed that TiO2 nanoparticles remained in the membrane during the process of membrane forming. 3.3. Anti-fouling performance In order to compensate for the difference in initial water flux (J0) of membranes, the ratio of the measured fluxes was chosen to represent anti-fouling and fouling mitigation performances. When comparing membrane filtration performance, the six important measures were the ratio of BSA solution flux to initial

Fig. 5. X-ray diffraction patterns of (a) TiO2 nanoparticles, and (b) PES membrane and (c) PES/TiO2 composite membrane.

water flux (Jp/J0), the fouling ratio (1  Jw/J0), the ratio of flux after rinsing (Jw1/J0) and back flushing with deionized water (Jw2/J0), the flux recovery ratio after water cleaning {(Jw1  Jw)/ (Jw2  Jw)}, and the flux recovery ratio after back flush {(Jw2  Jw1)/(Jw2  Jw)}. The BSA solution ratio (Jp/J0) measures the relative degree to which the membrane fouls during BSA solution filtration [13]. The closer this flux ratio is to unity, the lower the membrane fouling is during BSA filtration. The fouling ratio (1  Jw/J0), is the flux lost ratio to fouling. The less this value is, the better the anti-fouling performance of membrane is. The cleaning steps contained rinsing with deionized water and then back flushing with deionized water designed to remove surface fouling and interior pore fouling, respectively. The effectiveness of the cleaning was represented by the flux ratios {(Jw1  Jw)/(Jw2  Jw)} and {(Jw2  Jw1)/(Jw2  Jw)}. The filtration results for the PES membrane and PES/TiO2 composite membranes with different TiO2 content are shown in Table 3. The membrane foul decreases as the TiO2 content increased as shown by the increase in the value of Jp/J0, from 0.13 for the PES membrane to 0.45 for the PES/TiO2 composite membrane with 0.5 wt.% TiO2. This decreased tendency towards fouling is most likely due to the improved hydrophilicity resulted from the TiO2 addition. However, on increasing the TiO2 content to 0.7 wt.%, the value of Jp/J0 declined to 0.3. This result may have been induced by the blockage and the collapse of the membrane pore structure caused by excessive amount of TiO2. As the defective pore structure of the membranes was damaged easily by water pressure during the filtration process, the more of the BSA solute remained in the membrane pores. Therefore, more serious pore fouling occurred in the membrane with higher TiO2 content. This

Table 3 Flux changes during filtration and after cleaned TiO2 content (wt.%)

Jp/J0

1  Jw/J0

Jw1/J0

Jw2/J0

(Jw1  Jw)/(Jw2  Jw)

(Jw2  Jw1)/(Jw2  Jw)

0 0.3 0.5 0.7

0.13 0.16 0.45 0.3

0.77 0.71 0.48 0.65

0.40 0.45 0.65 0.48

0.69 0.80 0.94 0.86

0.37 0.32 0.31 0.24

0.63 0.68 0.69 0.76

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Fig. 6. SEM photographs of fouled and cleaned membranes. (a) Fouled membrane and (b) cleaned membrane; 1: top surface, 2: bottom surface, 3: cross-section.

result also demonstrates that the anti-fouling performance of the PES/TiO2 composite membrane with 0.5% TiO2 is better than others as shown in Table 3. The fouling ratio (1  Jw/J0), decreases initially to a minimum value 0.48 with the increase of TiO2 content to 0.5 wt.%, and then increases to 0.65 at 0.7 wt.% TiO2. Flux recovery ratio after rinsing and back flushing with deionized water, Jw1/J0 and Jw2/J0, increases with the increase of TiO2 content and reach a peak value at 0.5 wt.%, then fall when TiO2 content was further increased to 0.7 wt.%. This result indicated that the membrane with 0.5 wt.% TiO2 content is easier to clean than the others. The values of {(Jw1  Jw)/(Jw2  Jw)} and {(Jw2  Jw1)/(Jw2  Jw)} listed in Table 3 show that compared with the inner pore fouling, the surface fouling contributed only a small portion of the total fouling (Jw2  Jw), all values of {(Jw1  Jw)/(Jw2  Jw)} are lower than 0.4. As the TiO2 content increases, the surface fouling decreased and the proportion of inner pore fouling increased. As shown in Table 3, the {(Jw1  Jw)/(Jw2  Jw)} values gradually decrease from 0.37 to 0.24, while the (Jw2  Jw1)/(Jw2  Jw) values gradually increase from 0.63 to 0.76. This result indicates that the membrane fouling was caused mainly by the inner pores, and the recovery of membrane water flux was attributed to the cleaning of back flushing. Fig. 6 shows the SEM images of the fouled membrane containing 0.5 wt.% TiO2 after BSA solution filtration and the cleaned membrane after rinsed and back flushed by deionized water. As shown, the top surface of the fouled membrane was entirely covered by pollutant, and pollutant granules blocked the pores. Rinsing and back flushing almost cleaned out the pollutant on the surface entirely and the pore became clear. The crosssection image shows the inner fouling was also cleared effectively. This result indicates that the membrane was easily cleaned physically such as by rinsing and back flushing with water because of the improved hydrophilicity of the membrane. 4. Conclusion PES/TiO2 composite membranes were prepared by phase inversion method. Addition of TiO2 to the polymer casting solution affected the properties of the membrane. The main conclusions were listed as follows:

1. The structure of membrane was not obviously affected by addition of TiO2, but higher content of TiO2 in casting solution resulted in broken and collapsed pore structure. 2. The hydrophilicity, thermal stability and mechanical strength of membrane were enhanced through adding TiO2 nanoparticles. Higher TiO2 content resulted in decrease of mechanical strength because of the defective membrane structure. 3. XRD, TGA and mechanical strength analyses indicated that TiO2 nanoparticles were dispersed uniformly resulting in better compatibility between the polymers and TiO2 nanoparticles. 4. The appropriate addition of TiO2 improved the anti-fouling ability of the membrane. Acknowledgements This work was sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References [1] L. Ming-Liang, Z. Jian-Qing, T. Wu, P. Chun Sheng, Hydrophilic modification of poly(ethersulfone) ultrafiltration membrane surface by self-assembly of TiO2 nanoparticles, Appl. Surf. Sci. 249 (2005) 76–84. [2] J. Pieracci, J.V. Crivello, G. Belfort, Increasing membrane permeability of UVmodified poly(ether sulfone) ultrafiltration membranes, J. Membr. Sci. 202 (2002) 1–16. [3] F.G. Wilhelm1, I.G.M. Pu¨nt, N.F.A. Van Der Vegt, Cationpermeable membranes from blends of sulfonated poly(etherether ketone) and poly (ether sulfone), J. Membr. Sci. 199 (2002) 167–176. [4] I.C. Kim, J.G. Choi, T.M. Tak, Sulfonated polyethersulfone by heterogeneous method and its membrane performance, J. Appl. Polym. Sci. 74 (1999) 2046–2055. [5] M.L. Steen, A.C. Jordan, E.R. Fisher, Hydrophilic modification of polymeric membranes by low temperature H2O plasma treatment, J. Membr. Sci. 204 (2002) 341– 357. [6] Y. Yanan, W. Peng, Preparation and characterizations of a new PS/TiO2 hybrid membranes by sol–gel process, Polymer 47 (2006) 2683–2688. [7] Z.S. Wang, T. Sasaki, M. Muramatsu, Y. Ebina, T. Tanake, L. Wang, M. Watanabe, Self-assembled multilayers of titania nanoparticles and nanosheets with polyelectrolytes, Chem. Mater. 15 (2003) 807–812. [8] R. Molinari, M. Mungari, E. Drioli, A.D. Paola, V. Loddo, L. Palmisano, M. Schiavello, Study on a photocatalytic membrane reactor for water purification, Catal. Today 55 (2000) 71–78. [9] R. Molinari, C. Grande, E. Drioli, L. Palmisano, M. Schiavello, Photocatalytic membrane reactors for degradation of organic pollutants in water, Catal. Today 67 (2001) 273–279.

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[10] R. Molinari, L. Palmisano, E. Drioli, M. Schiavello, Studies on various reactor configurations for coupling photocatalysis and membrane process in water purification, J. Membr. Sci. 206 (2002) 399–415. [11] Kwang Man Kim, Nam-Gyu Park, Kwang Sun Ryu, Soon Ho Chang, Characteristics of PVdF-HFp/TiO2 composite membrane electrolytes prepared by phase inversion and conventional casting methods, Electrochim. Acta 51 (2006) 5636–5644.

[12] C. Xiaochun, M. Jun, S. Xuehua, R. Zhijun, Effect of TiO2 nanoparticle size on the performance of PVDF membrane, Appl. Surf. Sci. 253 (2006) 2003– 2010. [13] P. John, J.V. Crivello, B. Georges, Photochemical modification of 10 kDa polyethersulfone ultrafiltration membranes for reduction of biofouling, J. Membr. Sci. 156 (1999) 223–240.